There has been a number of new commentaries published on the book of Esther in recent years. Serious students of this book will especially benefit from the close reading of Esther provided by Berlin, one of favourites (I also really like her works on Hebrew Parallelism and Narrative). Bush and Levenson, offer similarly detailed analysis, as does Beal. In this regard, while not technically a commentary, I quite like Fox's Character and Ideology volume. On a popular front, Jobes, Roop, and Baldwin are all good Christian introductions to this book., while pastors will find much good comment in Bechtel.